The feds have traced the buyer and seller of the illegally owned weapons used by William Spengler Jr. in the ambush killing of two upstate firefighters, it was reported yesterday.

The Bureau of Alcohol, Tobacco, Firearms and Explosives said it has turned the information over to New York State Police, but couldn’t release details because the investigation is ongoing, The Associated Press said.

Spengler, 62, wasn’t able to legally purchase the .223 Bushmaster AR-15 assault rifle used in the slayings because he served 17 years for killing his grandmother.